Indoor air within and around enclosed environments, such as buildings, vehicles and structures, is affected by a plurality of substances comprising contaminants. Among these contaminants, often with the highest concentration, is carbon dioxide (CO2). There are other contaminants which may appear in relatively lower concentrations yet are no less important to monitor and/or reduce. A class of such contaminants is a group of species of organic vapors, broadly referred to as Volatile Organic Compounds (VOC). Contaminate gases (e.g., CO2) and VOCs, and corresponding vapors thereof, may collectively be referred to as a “gas(es)”. The sources of these contaminants include, inter alia, the human occupants themselves—from respiration and perspiration to clothing and cosmetics—as well as building materials, equipment, food and consumer products, cleaning materials, office supplies or any other materials which emit VOCs. Other classes of contaminants are inorganic compounds and microorganisms such as bacteria, viruses, mold, fungi and airborne particles. Additional gaseous contaminants may be sulfur oxides, nitrous oxides, radon, or carbon monoxide.